This invention relates to compounds useful as lubricants for polyethylene snow sliders, such as skis, snowboards and sleds. Lubricants of this type are of particular interest as ski waxes since they reduce friction between the polyethylene running surface of the ski and the snow, which results in higher skiing speeds.
A ski in motion possesses kinetic energy and the more of this energy it maintains, the faster it will move. Some of this energy is lost through friction and is converted to heat or is lost due to the vibration of the ski. Energy is also expended for plowing and compaction which occur when the snow is compressed or pushed aside as the ski is moving. The less energy a moving ski consumes through plowing and compaction, vibration and friction, the more kinetic energy--and consequently, speed--it retains.
There are several methods of minimizing kinetic energy loss. For example, to minimize kinetic energy loss due to plowing and compaction a race course is compacted mechanically prior to the race. Vibration, which is characteristic of the ski, is reduced by the proper utilization of ski construction materials.
Kinetic energy loss due to friction can also be minimized. The following friction components may be present in a glide situation:
1. Dry friction, which occurs in areas where dry snow particles touch the ski base;
2. Capillary suction, which occurs when free water is present and adheres to the base, producing a suction effect;
3. Friction due to the presence of dirt (i.e., diesel oil, pollen, rock-dust), which occurs when atmospheric contaminants adhere to the base and the snow at the same time, connecting them and reducing speed.
Two main methods have been used to reduce kinetic energy loss due to friction:
1. Base structuring: Various textures are imprinted on the ski bases by the manufacturers and they have a dual function:
They reduce capillary suction by preventing the formation of continuous water films.
They reduce the contact area between the base and the snow.
Ski bases are made from polyethylene, a polymer ideally suited for this use due to its following properties: Excellent elasticity over the required temperature range, low cost, availability in a variety of hardness grades, ease of repair, low water absorption, and good adhesion to hydrocarbon waxes. The major disadvantage of polyethylene is that it can be damaged if heated at temperatures approaching 120.degree. C.
2. Waxing: Ski waxes are solid lubricants that reduce friction between the ski and the snow. When selecting a wax, one optimizes the following four properties:
Hardness--The wax must always be harder than the snow so the snow does not penetrate it.
Friction coefficient--The friction coefficient must be as low as possible.
Water repellency (hydrophobicity)--Water repellency must be as high as possible.
Dirt absorption--The wax must not absorb dirt, pollen or oily atmospheric contaminants.
Until the mid-1980s, most ski waxes were hydrocarbons. Three types of hydrocarbons are typically used for ski wax production:
Paraffins (linear), which provide low friction coefficient.
Microcrystalline waxes (branched), which provide elasticity.
Synthetic (polyethylene or Fischer-Tropsch) waxes, which provide resistance to penetration by hard snow crystals.
Almost all ski waxes formulated for warm snow conditions are blends of soft paraffins and soft microcrystalline waxes. Blends of harder paraffins and microcrystalline waxes are used for more aggressive snow. Synthetic waxes are very effective wax blend hardeners so they are frequently added to wax formulations intended for use on very cold snow. Hydrocarbon ski waxes typically melt at 55.degree. C. to 95.degree. C. and are applied on the ski base as follows: A bar of wax is placed on a waxing iron which is heated to no more than 100.degree. C. and wax is dripped on the ski base. The iron is then used to distribute the wax uniformly on the ski base.
In the mid-1980s, perfluorocarbon ski waxes were developed. U.S. Pat. No. 4,724,093 describes perfluorocarbon lubricants containing from 10 to 20 carbon atoms and with melting points ranging from 36.degree. C. for C.sub.10 F.sub.22 to 110.degree. C. for C.sub.16 F.sub.34. Commercially available perfluorocarbon waxes are blends with average carbon lengths of 14 to 16 and have melting points of 95.degree. C. to 100.degree. C. It is well known in the ski wax industry that perfluorocarbon waxes with melting points lower than 90.degree. C. exhibit inferior performance. Perfluorocarbon waxes are applied over hydrocarbon base waxes and offer outstanding performance on wet and relatively new snows. Their good performance is attributed to their high degree of water repellency, which reduces capillary suction. Perfluorocarbon waxes also resist oil and dirt, which is attributed to their very low surface energy. This results in reduced friction on dirty snow.
Although perfluorocarbon waxes exhibit outstanding performance on wet and nonaggressive snows, they lack mechanical strength and are easily penetrated by aggressive snow crystals. As a result, skis prepared using perfluorocarbon waxes may demonstrate reduced skiing speed at snow temperatures below -8.degree. C. Furthermore, perfluorocarbon waxes perform poorly under low humidity conditions where the water content of the snow is very low.
The application of commercial perfluorocarbon waxes also presents a problem. Their high melting points necessitate a recommended ironing temperature of 150.degree. C. This may damage the base structure and increase friction. Furthermore, perfluorocarbons tend to sublime under these ironing conditions and this may present a significant pulmonary risk to the ski technician. Many technicians have therefore resorted to applying perfluorocarbon waxes by rubbing them on the ski base. This significantly limits durability and it is not uncommon for perfluorocarbon waxes applied by rubbing to stay on the ski base for less than several hundred meters. This can be particularly problematic during long distance cross country racing where races can be more than 20 kilometers long.
European Patent Application 0 421 303 A2 describes fluoroalcohols, fluoroesters and polyfluoroalkyl ester copolymers for use as lubricants for skis. The lubricants for skis described in the European Patent Application are not suitable for snow temperatures below -10.degree. C.
International Application WO 94/11468 describes perfluoroalkyl terminated urethane lubricants. The compounds claimed in International Patent WO 94/11468 exhibit poor performance compared to commercial perfluorocarbon lubricants, as measured by skiing speed.